1. Field of the Invention
The invention relates to filters and filter units.
2. Review of the Prior Art
In known filters, a number of filter units are formed together in a stack to provide a filter having a required surface area of filter medium. An example of this is shown in GB-A-2063089A.
This specification discloses a filter comprising a number of filter units which are formed by generally circular plastic supports with filter media bonded to their upper and lower surface. Radial passages in the support lead to a central aperture which, when the filter units are stacked together, forms an outlet for filtrate. The filter units are contained within a casing which provides an inlet for fluid being filtered.
It is a disadvantage of the arrangement of GB-A-2063089 that the filter units are complicated to manufacture and difficult to assemble. The flow is not fully reversible in this arrangement and the filter medium can only be encapsulated by the provision of a separate casing.
GB-A-1234283 shows a similar arrangement in which a disc-shaped holder carries filtering material. Each disc has peripheral apertures and a central aperture so that when the discs are stacked together, fluid to be filtered enters via the peripheral apertures and passes generally radially to the central aperture via the filtering material.
Again, these discs can only be encapsulated by the provision of a separate casing because of the peripheral apertures. The provision of such a casing adds to the cost and lengthens the manufacturing process.